P70-6.
PAINT PERFORMANCE ON FOREST PRODUCTS
Modification of wood and plywood to improve paintability
May 1952
C'Pr
No. 1926
UNITED STATES IDEPARTMENT OF AGRICULTURE
LE.ORESTS ERVCE
I
FOREST FYODUCTS LAB ORATOR Y
Madison 5, Wisconsin
In Cooperation with the University of Wisconsin
PAINT PERFORMANCE ON FOREST PRODUUTS1
MODIFICATION OF WOOD AND PLYWOOD TO IMPROVE PAINTABILITY
By
F. L. BROWNE, Chemist
and
D. F. IAUGHNAN, Technologist
Forest Products Laboratory,2 Forest Service
U. S. Department of Agriculture
ecImMGOV.
The performance of paint on ordinary lumber and plywood has been studied in
detail at the Forest Products Laboratory since 1924. Most of the studies
concerned house paints because they usually wear out in 4 or 5 years. Interior coatings may last for many years unless, like floors, they are subject
to severe mechanical wear. Interior and exterior coatings are affected by
wood characteristics in much the same way except that more concern may be
expressed about surface imperfections, such, as raised grain and wood checks,
in connection with interior than with exterior finishes.
Results of the studies of house paints on ordinary lumber of all of the
commercially important native woods have been published repeatedly (9, 11,
13, 14, 22)el Plywood has essentially the same painting properties as
lumber of the kind of wood with which the plywood. is faced. Some plywood
is reputed to be more seriously given to face checking than is lumber of
the same kind of wood, but, in part, that reputation may be due to the fact
that wood checks are more displeasing on the large surfaces presented by
plywood than they are on relatively narrow boards.
Presented before the Pacific Northwest Division of the. Forest Products
Research Society May 20, 1952, at Portland, Oreg.
2Maintained at Madison, Wis., in cooperation with the University of
Wisconsin.
.Numbers in parentheses refer to literature cited at end of article.
Agriculture-Madison
Rept. No. D1926
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Although all commercially important softwoods can be kept well painted
economically, some of them are much more exacting than others in their requirements of priming procedures, paints, and maintenance programs to that
end. Only 10 percent of the annual cut of softwoods is of the lightweight
species easiest to keep well painted, and nearly 70 percent is of the
heavy species that require greatest care. Moreover, only a fraction of
the production of softwoods is of the clear grades suitable for fully satisfactory painting.
Many studies have been made to find better ways of painting the more exacting woods. Special priming paints were discovered that go a long way
toward making the heavier softwoods hold paint as well as the lightweight woods do (3, 6, 7, 12, 17).
As far back as 1929, at a wood-painting conference at the Forest Products
Laboratory, representatives of the paint industry suggested that modification of the wood should be considered in an effort to improve paint performance on the more exacting woods; they considered it unfair to hold paint
responsible for making all of the modifications needed. Efforts to alter
wood surfaces were started soon after, but progress with them was slow.
Wood surfaces are much less amenable than paint to practicable modifications. As it turned out, some of the most fruitful alterations of wood
surfaces had to await the development of new auxiliary materials.
Treatment of Wood Surfaces with Chemicals
The studies of paint performance on different woods gave evidence that
extractives in some of the woods that are easiest to keep painted,
particularly in redwood and southern cypress, exert a favorable effect
on the durability of paint (14). Accordingly, tests were started in 1930
in which the aqueous extract of redwood heartwood and the alcoholic extract of cypress heartwood were transferred to the surfaces of boards of
eastern hemlock, which were then tested for painting properties. The
treated boards gave better paint performance than matched but untreated
boards (9). The extent of the improvement, however, was relatively
small. The beneficial ingredients in the cypress and redwood extracts
are probably antioxidants for linseed oil; in moderate proportions they
may retard the embrittlement of paint as it ages. But antioxidants cannot be used in larger proportions for fear of preventing the initial drying of paint, Even the proportions normally present in redwood and
cypress seriously retard the drying of many paints under the unfavorable
conditions of high humidity, low temperature, and absence of sunlight (2).
In 1932 tests were started in which lumber was treated superficially
with reactive gases or with aqueous solutions of inorganic chemicals
that alter the pH and might alter the surface reactivity of cellulose and
lignin. It was admittedly "shot-gun" research without much clear scientific guidance. The hope was that one or more of the treatments might
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improve the adhesion and durability of paint. None of the treatments, however, improved paint performance; most of them had little or no effect and
some impaired paint durability. Perhaps the outcome should have been expected from an understanding of the nature of adhesion of paint to wood (2)
for any effect of chemically altering the cellulose and lignin in wood surfaces would have to do primarily with specific adhesion of paint to wood.,
which is believed to be very good as long as the paint is young but to be
gradually lost as the paint becomes embrittled with age (8). Adherence of
old paint is thought to be strictly mechanical. If so, the modifications of
wood surfaces necessary to improve mechanical adhesion would have to be on
a macroscopic or a microscopic scale rather than on a molecular scale.
Later in this paper it will be shown that mechanical adhesion of aged paint
to the heavier softwoods can be improved by suitable coarse roughening of
the surfaces.
Dimensionally Stabilized Wood
Since old, brittle paint adheres to wood mechanically, it is reasonable
to suppose that actual breaking away of the coating is hastened by alternate swelling and shrinking of the wood fibers at the interface in response to changes in moisture content. Paint durability, then, should be
improved by treating the wood in such a way as to render it more nearly
stable, dimensionally; that is, by antishrink treatment, provided that the
treatment is not harmful to paint in some other respect. Several methods
of stabilizing wood have been developed (19), though none of them is as
yet inexpensive and convenient enough to have attained widespread use.
Amtylated Wood
Acetylated wood (20), in which hydroxyl groups of the lignin and cellulose
have been esterified with acetiz.. anhydride to the extent of approximately
20 percent by weight, has only 30 percent of the shrinkage of normal wood.
Accordingly, preliminary tests were started in 1947 to determine the effect
of acetylation on paint performance.
Test panels were 12 by 16 inches in size, made of five-ply, resin-bonded
plywood with core and crossbands of Douglas-fir and 1/32-inch face veneers
of sweetgum veneers. Panels were made in pairs in which the face veneers
of one panel were acetylated and those of the other panel were left untreated for comparison. Twelve pairs of panels were faced with sapwood of
sweetgum and four pairs with heartwood of sweetgum. The treated sapwood
contained 20 percent of acetyl groups, and the treated heartwood 18 percent. One pair of panels with sapwood faces was exposed without any
coating.
Table 1 records the paint, enamel, or lacquer coatings tested and the
extent to which the performance of each coating proved better on acetylated
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than on untreated wood. The meaning of the symbols for concisely indicating
the composition of the coatings is explained in the appendix to this report.
All coatings of paint or enamel consisted of one application of priming paint
or of enamel undercoater followed by two applications of finish paint or
enamel. Lacquers were applied by spraying until satisfactory hiding was obtained. The test panels were exposed to the weather in the vertical position,
facing south, at Madison, Wis., in March 1947 and were last inspected in
September 1951, about 4-1/2 years later.
On 2 pairs of panels out of the 15 pairs no difference in performance of the
coatings had appeared up to the last inspection, and the coatings remained
in good condition on both treated and untreated wood. On the other 13 pairs
of coated panels the performance of the coating proved better on the acetylated than on the untreated wood. Sometimes, as with paint L and with
enamel T139 (pre), the improvement from acetylation was slight, though it may
become more marked as the exposures continue to ultimate coating failure.
But with other coatings, such as enamel (TZ 15 ) 325 (ar) and with the lacquers
performance was very much superior on the acetylated than on the untreated
wood. Flrt evidence of such superiority sometimes appeared as early as
November 1948, after 1-1/2 years of exposure. Figure 1 shows some of the
results.
Acetylation affected the performance of coatings only in matters of coating integrity, such as cracking, curling, flaking, and scaling. Matters
of coating appearance, such as gloss, color, collection of dirt, and susceptibility to mildew, were not affected by acetylation, nor were chalking and erosion as far as could be determined. The pair of panels exposed
without any coating proved that acetylation markedly delayed and reduced
wood checking and greatly retarded the graying of the wood from weathering.
Acetylation, therefore, offers great promise of improving paint performance
and minimizing wood checking. A need for further experiments with such
woods as Douglas-fir and southern yellow pine, both as plywood and as
lumber, is clearly indicated.
Impreg
Another method of improving the dimensional stability of wood is that of
impregnation with resin-forming materials followed by curing of the resins
in place in the wood structure. The product is known as impreg (18).
Preliminary exposure tests of the painting of impreg were started in 1937
and were continued until December 1944.
The test panels were 3/8 by 18 by 18 inches in size, made of three plies
of 1/8-inch Douglas-fir veneer. Six panels were made with untreated face
plies and six with face plies of impreg produced from the Douglas-fir
veneer. All of the panels were bonded with hot-pressed phenolic-resin glue.
Before painting, the bottom half (9 by 18 inches) of each impreg-faced
panel was thoroughly sandpapered to remove any continuous layer of resin
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and to permit contact of the paint with impregnated wood fibers. One
plain- and one impreg-faced panel were coated on all surfaces and edges
with one of each of the following finishes:
1.
2.
3.
4.
5.
6.
Three coats of paint L p/nv 0.27.
One coat of aluminum house paint plus 2 coats of paint L
p/nv 0.27.
p/nv 0.29.
Three coats of paint (TL 28Z—)
20 116
One coat of aluminum house paint plus 2 coats of paint
(TI, 2 8Z-2
0 ) 116 p/nv 0.29.
One coat of paint L, plus 1 coat of a mixture of equal parts
of paint L and enamel (TZ66)207(e)) p/nv 0.19, plus 1 coat
of enamel (TZ66)207(e) p/nv 0.19.
One coat of aluminum house paint, plus 1 coat of the mixture
of paint L and enamel (TZ66 ) 206(e), plus 1 coat of enamel
(TZ66)207(e) piny. 0.19.
The painted panels were exposed to the weather in the vertical position
facing south.
There were both favorable and unfavorable developments in paint performance
on the impreg as compared with plain Douglas-fir. There was little or no
crumbling or flaking of paint from the bands of summerwood in impreg even
after 7-1/2 years of exposure. On untreated Douglas-fir, paint L, when
self-primed, began to crumble from summerwood after 3-1/2 years and left
the summerwood almost entirely bare within 7-1/2 years. Paint (TL28Z28) 116,
when primed with paint L, began
when self-primed, and enamel
(TZ66)2o7(e)'
to flake from the summerwood of untreated Douglas-fir during the fourth and
fifth years. But on impreg, paint L showed little crumbling and paint
(TL2 8Z 28) 11 6 and enamel (TZ66)207(e) showed no flaking over summerwood after
7-1/2 years. Use of aluminum primer on plain Douglas-fir also succeeded
in preventing crumbling or flaking from summerwood, though no more effectively
than the impreg did.
Both of the paints and the enamel, however, developed their normal checking
or cracking patterns just as early over impreg as they did on plain Douglass
fir. But on impreg the checking or cracking led to alligatoring during the
fourth year with the enamel or the fifth year with the two paints. With
paint L the alligatoring was confined to the summerwood bands in the impreg.
Alligatoring occurred earlier and became much more pronounced when paint
(TL2 8Z 28) 11 6 or the enamel were applied over aluminum primer on impreg
than when they were applied over white primer. Thus the beneficial effect
of impreg in preventing crumbling and flaking waa partly offset by a tendency
to cause alligatoring. It is possible, however, that the finishing systems
were more than usually prone to alligatoring in consequence of their application indoors without exposure to sunshine b tween coats, for paint
(TL28428)116 eventually became distinctly alligatored over aluminum primer
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on untreated Douglas-fir. Nevertheless, the alligatoring in the 1937 tests
on impreg should be kept in mind in connection with the alligatoring reported later on in the 1941 tests on plywood covered with resin-impregnated
paper pulp.
Sandpapering the surfaces before painting exerted little or no effect on
the performance of the coatings. On two or three of the panels there was
slightly less alligatoring over the sandpapered than over the unroughened
part of the surface, but the difference was too slight to be considered
significant.
No wood checking ever appeared in the impreg-faced panels. The plain Douglasfir faces, however, developed a moderate degree of wood checking during the
course of the tests.
Figure 2 shows how the impreg prevented crumbling and flaking from summerwood. It also shows alligatoring over the impreg not found on the comparison panels of plain Douglas-fir.
uer Covering
In 1932, following a proposal by M. E. Dunlap of the Forest Products
Laboratory, tests were started in which 6-inch bevel siding of southern
yellow pine was covered on all six sides with paper. To make the test
panels, 6-foot boards were cut in half (to 3-foot lengths) and paper glued
to one half but not to the other half. Thus panels of covered and of
ordinary siding were assembled in which the wood vas carefully matched.
Two weights of paper were tested, one a thin, unsized tissue paper and the
other a fairly heavy kraft wrapping paper. The paper was glued in place
with water-resistant animal glue of the paraformaldehyde formula (1),
inasmuch as resin glues had not yet been developed.
A covered and an uncovered test panel were painted with three coats of
paint L p/nv 0.27, and another pair of panels was painted with paint
(TZ23 ) 92 p/nv 0.29. During the painting the panels were exposed to sunshine for 8 hours between coats of paint. After painting the panels were
exposed at 45 degrees from the vertical facing south on the test fences
at Madison, Wis. The exposures lasted from August 29, 1932 to July 8,
1935.
The results are summarized in table 20 Paint was applied slightly more
generously over paper of each kind, making slightly thicker coatings,
than over uncovered wood. The coatings were somewhat thicker than 0.005
inch,generally considered optimum for initial paint jobs on previously
unpainted wood. As a result the coatings were distinctly more durable
than is normally expected on southern yellow pine exposed at 45 degrees
facing south, but when flaking or scaling began the stresses operating
at the paint-paper interfaces may have been abnormally high.
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Figure 3 shows the panels painted with pure white lead paint at the conclusion of the tests. The covering of kraft paper succeeded in preventing
early crumbling from the summerwood, which can be plainly seen on the
uncovered panel, and thereby materially prolonged the durability of the
coating. Similar results were obtained with paint (TZ 23 ) 92 . The tissue
paper, however, proved too weak to withstand the shrinking of crumbling or
scaling paint after it has checked or cracked and started to break away.
The kraft paper, moreover, kept the exposed surfaces free from observable
wood checking and reduced the amount of cupping of the boards under the
severe weathering conditions of the test.
The test panels were removed from the fence in 1935 and stored in a laboratory, kept warm but not humidified in winter, until April 1937. Drying
of the wood led to shrinkage, which partly loosened the glue bond between
kraft paper and wood and left the paper slightly rumpled. The panels
covered with kraft paper and their matched but uncovered panels were then
repainted with two coats of fresh paint of the kinds used originally and
were restored to the test fences, but this time in the vertical position
facing south. The new exposures continued until the summer of 1944. Reabsorption of moisture out of doors soon swelled the wood sufficiently to
draw the kraft paper taut again, but contraction of the paint coating as
it aged cracked the paper over the areas where the glue bond had broken.
Paper and coating then began to curl and scale from the wood. On the
other hand, where the paper remained firmly glued to the wood, it continued
to improve paint durability by preventing crumbling or flaking from the
summerwood bands.
The 1932 tests showed clearly that a covering of paper of sufficient strength
would greatly improve the performance of paint on the heavy softwoods provided that the paper could be held firmly in place by more weather-resistant
glue than animal glue treated with paraformaldehyde.
Tests in 1949
In 1949 further studies were made with paper coverings on southern yellow
pine. Paper wasglued to both faces of boards 25/32 inch thick and 6-1/4
inches wide.,t The boards were then resawed and edge-jointed to make
nominal 6-inch bevel siding with paper glued on the exposed face only.
Preliminary tests showed that ordinary paper gave trouble in jointing or
crosscutting because the paper became badly frayed at the cut edges. Kraft
paper impregnated with 5 percent by weight of water-soluble phenolic resin
and cured without pressure at 325° F. for 15 minutes, when glued to the
boards permitted cutting to smooth edges without any fraying.
lI Bruce G. Heebink designed and prepared all of the paper-covered boards
for these tests.
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For the tests two thicknesses of treated kraft paper were used on different panels, namely, 2 mils and 6 mils. For each thickness of paper, test
panels were made with each of three different glues to bond the paper to
the wood, namely, casein glue, urea-resin glue, and a phenolic-resin glue
of the type curing at room temperature. For each combination of thickness
of paper and kind of glue, one test panel was exposed on the test fences
without painting and one panel was painted on the exposed face. There were
also two panels of southern yellow pine without covering of paper, one
painted and one allowed to weather without painting. All painting consisted
of a priming coat of paint (TL25 ) 79 p/nv 0.35 and one finish coat of paint
(TZ20Z20 ) 114 p/nv 0.30, which made coatings approximately 3.4 mils thick.
The panels were exposed vertically, facing south, from July 27, 1949 to
October 31, 1950.
Within 2 or 3 weeks after starting the exposures, some of the panels covered
with casein-glued paper, both painted and unpainted, began to show blisters
where there was separation at the wood-paper interface. There was also
BOMB separation of paper from wood along the bottom edges of boards. Such
early failures are attributed to inadequate gluing; probably to unduly
long assembly time in the gluing operation. By October 1950, after 14-1/2
months, all unpainted panels with paper covering blistered and loosened
at bottom edges; those with casein glue were most seriously and those with
phenolic-resin glue least seriously damaged. The painted panels with paper
covering glued with either urea-resin or phenolic-resin glue were still in
excellent condition.
The painted panels with paper glued with urea-resin or phenolic-resin
glue and the painted panel without paper were reassembled and restored to
the test fence in the spring of 1951. By May 1952, after a little more
than 2 years of total exposure, there was blistering on all of the papercovered panels. On the control panel without paper the paint was beginning
to flake from summerwood. On paper coverings there was no paint flaking.
The 2-mil paper did not succeed in hiding raised grain, but the 6-mil paper
allowed no raised grain to show.
Paper covering for the exposed faces of siding, if about 6 mils thick,
shows promise of improving paint performance on the heavier softwoods,
but more work is needed to develop the technique of gluing the paper in
place reliably.
Coverings of Resin-Impregnated Pulp_
21:29221:
In 1941 tests were started with a commercial product consisting of exterior-grade plywood of Douglas-fir faced on both sides with a covering
said to be pulpboard impregnated and glued to the plywood with phenolic
resin. The pulpboard may have been somewhat compressed in the hot-press
gluing, but it was by no means saturated with resin and remained somewhat
porous, though not so absorptive of liquids as wood is. The covering was
1/32 inch thick.
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Test panels were 2 feet by 6 feet in size but were marked off into test
areas 2 feet by 1-1/2 feet in size. Panels were mounted in the vertical
position on both the north side and the south side of the test fence so
that the backs, which remained unpainted, were not exposed to the weather.
The upper half of the exposed face of each panel was sandpapered enough
to roughen the surface slightly and to remove any continuous film of resin
.that might have been at the surface. On any one panel each test area received a different priming coat or combination of priming coat and undercoat, but all four test areas of any one panel received the same finishing
paint or enamel. During painting the panels were exposed to sunshine for
at least 8 hours between coats. The panels were exposed on the test fence
at Madison, Wis., on June 3, 1941, and the final inspection was made on
August 25, 1944.
It was found that priming paints spread about 15 to 20 percent farther on
the surfaces of resin-impregnated pulpboard than they did on wood. The
difference arose from less absorption of paint liquids by the impregnated
surfaces. The thickness of dried coating formed was probably about the
same as on wood.
Sandpapering the surfaces before painting usually exerted no effect on
the 'performance of the coating, but with 8 of the 24 coating systems
tested the durability was impaired, sometimes seriously, by sandpapering.
Table 3 lists the coating systems tested and the important observations
of their performance. Unpigmented primers such as wood sealer and spar
varnish had markedly adverse effect on the durability of the paints and
enamels. Aluminum house paint as a primer gave the best performance with
the two house paints and one of the enamels, and one of the best with the
second enamel. The presence of any zinc oxide in a primer proved distinctly
harmful to the durability of any of the paints or enamels applied over it.
Figures 4 and 5 show some of the results. The most significant development
in the tests, however, was the tendency for many of the coatings to become alligatored, sometimes seriously so. With all of the coatings tested,
alligatoring must be considered an abnormality of performance. Yet only
three of the systems (both of the enamels when applied over aluminum
primer and enamel T4 20 (ar) when self-primed) remained entirely free from
alligatoring. Presence of zinc oxide in a primer under either of the
house paints made alligatoring especially conspicuous. Sandpapering before painting either failed to alter or increased the tendency toward
alligatoring.
Since the abnormality of alligatoring had appeared previously in resinimpregnated surfaces in the 1937 tests of impreg, there seemed to be a
strong presumption that the impregnating resin was responsible for it.
Incompatibility between the resin and paint coatings seemed to be indicated. The findings that the antagonism was worse toward house paints,
with oil vehicles, than toward enamels, with resinous vehicles, and that
presence of zinc oxide in a primer was an aggravation, were consistent
with that view. Nevertheless, judgment should be reserved in the light
of tests described farther on.
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Although no further tests have been made at the Forest Products Laboratory
with the resin-impregnated pulpboard surfacing, one of the authors (F0 L.
Browne) has seen adequate tests made by a paint manufacturer's laboratory
in the Chicago area with the same brand of covered plywood as manufactured
since the close of the war. Paint and enamel systems reasonably similar to
those of the Forest Products Laboratory tests were used. No difficulty
with alligatoring was experienced in the more recent tests. It is not known
whether the covered plywood had been changed in any way, such as by the
amount or kind of resin used, by the curing conditions, or by any lubricants
used on the press Gauls.
Papreg Tests
A wartime study of problems in aircraft finishing, started in 1943, involved comparative tests of 44 different finishing systems on birch plywood, poplar plywood, and on papreg, which is a resin-impregnated paper
laminate containing about 35 percent of phenolic resin (16) and compressed
to a smooth surface that is practically nonabsorptive of paint liquids.
Before applying finishes, half the area of each test panel of papreg was
sandpapered enough to destroy the glossiness and to make the surface slightly
rough.
The finishes applied were all commercial products, most of which presumably
conformed to currently applicable military specifications of the performance
type, but the composition of the finishes was only partly disclosed. They
included oleoresinous enamel systems, nitrocellulose lacquer systems, and
nitrocellulose dope systems with and without fabric. In each class there
were sealers, surfacers, and pigmented finish coats that were tested in
several combinations. Both glossy and lusterless (camouflage) finish coats
in yellow, dark-blue, aluminum, and olive-drab colors were included. There
were no house paints.
The outcome of the tests failed to indicate any incompatibility between
papreg and the finishes of aircraft type. Abnormal forms of failure such
as alligatoring or coarse scaling were almost entirely absent. In general,
the ultimate form of failure of each coating was essentially the same on
papreg and on plywood, though the rate at which failure took place varied
among the substrates.
As a rule, the finishes lasted longer on papreg than on yellow-poplar
plywood and longer on yellow-poplar than on birch plywood. According to
a scoring system adopted for the purpose in which the highest number represented the longest life of the coating, the average score for all finishes
on papreg was 4.7, on yellow-poplar 3.6 9 and on birch 3.0. Oleoresinous
finishes outlasted lacquer finishes, but the difference between them was
much less on papreg than on plywood. Thus the scores for oleoresinous
finishes on papreg and on yellow-poplar were 4.8 and 4.3, respectively,
whereas for lacquer finishes the corresponding scores were 4.0 and 2.5.
Sandpapering the surface of papreg had little effect on the performance
of finishes. For 34 finishing systems there was no observable effect of
sandpapering, for 5 systems durability was slightly impaired, and for 5
it was slightly improved by sandpapering.
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Resin-Impregnated Paper Covering (1944)
es of tests was started in 1944 on Douglas-fir plyA comprehensive
wood with and without covering on both faces with resin-impregnated paper.
The covering consisted of three plies of paper containing 40 to 50 percent of water-soluble phenolic resin, subsequently cured and compressed to
a thickness of 00009 inch, and having a weight of 80.pounds per thousand
square feet of single surface. The press cauls used produced a surface of
very low gloss and a slight degree of roughness. The covering material
used was a commercial product, the manufacturer of which supplied both the
covered and uncovered plywood for the tests.
The size and arrangement of test panels on the test fence at Madison, Wis.,
were essentially the same as has already been described for the 1941 tests
of resin-impregnated pulpboard covering, except for a much greater number
of panels and for repetition of all tests on plain plywood as well as on
the covered plywood. There were 320 test areas, 160 of which were plain
plywood and 160 were plywood covered with resin-impregnated paper. Eighty
test areas of each kind were exposed facing south and 80 facing north. A
total of 77 finishing systems were tested, of which 33 were house-paint
primers and house paints, 36 were oleo resinous enamels with various primers
and undercoaters, and 8 were nitrocellulose lacquer systems. For the
present purpose it is sufficient to report results with 18 of the housepaint systems and 10 of the enamel systems in table 4. With respect to
the nature of the differences in performance on plain and covered plywood,
the unreported finishing systems behaved like those reported.
The upper halves of all test areas of plywood covered with resin-impregnated
paper were sandpapered before any finishing material was applied. As a
rule, such roughening of the surface produced no noticeable difference in
the performance of the coatings on exposure. On 6 of the 80 test areas
of covered plywood facing south the coating failed slightly more rapidly
on the sandpapered portion than it did on the unroughened portion. All
of the tests of resin-impregnated surfaces, whether wood, puipboard, or
paper, indicate that sandpapering seldom proves helpful, usually has little
effect, and when it does is more often harmful than beneficial.
The upper halves of all test areas of plain plywood in the 1944 tests were
treated with a water repellent before painting. The water repellent was
a solvent naphtha solution of 105 percent of paraffin wax and 13.5 percent
of resin by weight. The treatment failed to improve the performance of.any
of the finishing systems tested. It likewise failed to effect any noticeable reduction in the amount of face chocking of the plywood during the
course of the tests.
During the painting it was found that the first coat of finishing material
applied, whatever its nature, usually was spread farther on the nonabsorptive surface of the covered plywood than it was on plain plywood. The
average spreading rate for all first coats was 600 square teet per gallon on
covered and 900 square feet per gallon on min plywood. Thus the covered
plywood requires only two-thirds the material needed for plain plywood.
When the thicknesses of dried coating were calculated, assuming no absorption of primer liquids by the resin-impregnated paper, the average for all
first coats was 1.1 mils for both the plain and covered plywood. The
difference in consumption of priming material, then, seems to be wholly the
quantity absorbed by plain wood.
The average spreading rates for all second coats were 611 and 653 square
feet per gallon on plain and on covered plywood, respectively. The corresponding averages for all third coats were 722 and 713. Thus in applying
coatings about 4.5 mile thick, there was a saving of a little less than 15
percent of finishing material for the covered plywood as compared with plain
plywood.
The covering of resin-impregnated paper presented a smooth, uniform surface
on which enamel finishes appeared to best advantage; on plain plywood the
appearance of the enamels was often impaired by slightly raised grain in
the wood, which formed noticeable ridges in the enamel surface over some
of the bands of summrwood. With paint finishes such effects of slightly
raised grain were usually made inconspicuous by the pattern of brush marks
left in the surface of the coating.
Table 4 shows that nearly all coatings proved more durable on the covered
than on the plain plywood. Only the southern exposures are reported because in 6 years the coatings facing north did not deteriorate sufficiently
to estimate their relative durabilities. The advantage for the covered
plywood was usually greatest for the least durable coatings. Thus the
systems consisting of sealer followed by two coats of house paint lasted
about 20 months longer on covered than on plain plywood. Where coatings
were durable for 70 months or longer on plain plywood, the durability on
covered plywood could hardly be more than a few months longer. The
average durability of 33 house-paint systems was 58 months on plain plywood
and 74 months on covered plywood. For 36 enamel systems the average durability was 63 months on plain and 77 months on covered plywood. For 8
lacquer systems the average durability was 51 months on plain and 71 months
on covered plywood.
Particularly significant is the fact that the difference between the most
durable and the least durable coating on covered plywood was much less
than the corresponding difference on plain plywood. In other words, the
covered plywood proved materially less sensitive to variation in the
quality of the coating than was plain plywood. A few of the most durable
finishing systems on plain plywood, particularly those in which aluminum
priming paint was used, compared favorably in durability with the best
systems on covered plywood.
The optimum pigment volume for priming coats in house-paint systems turned
out to be somewhat higher for covered plywood than for plain plywood. On
covered plywood pigment volume 0.4 generally gave best results, but on plain
plywood 0.3 or sometimes even 0.2 proved better. Such difference was to be
-12-
expected because wood absorbs much of the liquid from priming coats and
thereby raises the pigment volume in the dried coating, whereas on resinimpregnated paper there is no such absorption. For enamel systems special
primers or undercoaters usually proved advantageous for plain plywood, but
on the covered 'Arwood the simpler praptice of priming with the same enamel
used for the finish coat often proved better.
Figure 6 shows how coating on plain plywood eventually wore out by crumbling
or flaking from summerwood until the grain pattern of the wood was clearly
revealed. On covered plywood, shown in figure 7, crumbling or flaking set
in irregularly, following no pattern related to the grain of the wood or
the structure of the covering. In general, each paint or enamel followed
its own characteristic course of disintegration by checking and crumbling
or by cracking, curling, and flaking, whether it was on plain plywood or
on covered plywood. The covering, however, succeeded in preventing the
unduly early onset of crumbling or of flaking from summerwood that is so
troublesome on the heavier softwoods.
In the 1944 tests on resin-impregnated paper coverings none of the paints,
enamels, or lacquers tested developed abnormal alligatoring such as was
observed in the 1937 tests on impreg and the 1941 tests on resin-impregnated
pulpboard coverings. This was true even though a number of the priming
paints or enamel undercoaters contained zinc oxide. The alligatoring of
coatings in the 1937 and 1941 tests, therefore, cannot be attributed to the
mere presence of resin in the surface or of zinc oxide in the priming coat.
The alligatoring that has been observed in certain tests remains for the
present unexplained. Until more is learned about it, resin-impregnated
surfaces must be tested for paint performance on them to see whether difficulties with alligatoring may be experienced. The Forest Products Laboratory has more recent tests underway on paper coverings impregnated with
varying proportions of different resins, but the work has not yet advanced
sufficiently to warrant, detailed description.
Resin-Bonded Sawdust Covering
Tests were started in 1951 on a commercial product that consists of
Douglas-fir plywood covered on one face only with a finely granular material
said to be finely ground wood waste bonded together and to the plywood with
resin. The surface presented is light in color, lusterless, and slightly
rough to feel. It was found to be much more absorptive of paint liquids than
plain plywood.
When oil-rich house paints were thinned with extra linseed oil or paint
thinner for application on the covered plywood, the granular covering absorbed liquid so rapidly and extensively that the paint became dry under
the brush and was exceedingly difficult to spread evenly. Brushing had
to be done very quickly. Excessive quantities of priming paint were consumed, up to half again as much as was needed for plain plywood. Moreover,
such primers failed to seal the surface adquately to permit one subsequent
-13-
coat of finish paint to dry with uniform gloss. A second finish coat, the
third coat in all, dried with uniform gloss.
Special house-paint primers made with a substantial proportion of bodied
oil, which provides the property commonly called controlled penetration,
proved reasonably satisfactory on the granular covering, though it was still
necessary to spread the paint rapidly. The extra consumption of special
priming paint was only 3 to 15 percent of that needed on plain plywood. One
coat of house-paint primer sealed the surface well enotigh for one coat of
finish paint to dry with uniform gloss.
Covering the plywood on one face only produced an unbalanced construction
that led to marked cupping of the test panels while they were in process
in the carpenter shop and painting laboratory. The covered face became
concave. When the panels were straightened by nailing them in place on the
test fence, a number of conspicuous cracks developed in the granular covering Dnd the. coating of paint over it. It would be advisable to balance the
construction of the product to avoid such difficulties, because precise
control of moisture content at all times during building construction cannot be expected.
The tests have not yet been exposed long enough to give significant indication of the performance of the coatings.
Mechanical Treatment of Wood Surfaces
The chief shortcoming of the heavier softwoods comes from the wide bands
of summerwood presented for contact with the paint coating. Edge-grain
boards, in which the bands of summerwood are of minimum width, hold paint
materially longer than otherwise similar flat-grain boards. Better paintability of flat-grain boards should be expected if a mechanical processing
of the surfaces can be devised to subdivide the wide bands of summerwood.
Douglas-fir plywood with striated surfaces, such as are shown in figure %,
has been on the market for some time. From observations of practical experience like that shown in figure 8, it is evident that the striations
succeed in improving paint performance by narrowing the bands of summerwood in contact with the paint. In addition, the process very significantly
reduces face checking in Douglas-fir plywood.
In 1951 the Forest Products Laboratory began tests of striations on lumber
siding of Douglas-fir and of southern yellow pine. The tests are not yet
old enough to tell whether an improvement in paint performance has been
effected. They have shown, however, that when siding is erected with the
striations horizontal, the paints often become more seriously soiled with
dirt than they'do on smooth lumber or on striated lumber erected vertically,
Further tests would be highly desirable in which other patterns of striations. would be tried, particularly if some mechanical method of narrowing
-14-
the bands of summerwood without so greatly altering the appearance of the
painted wood could be found,
Conclusion
The heavier softwoods, which are the most abundantly available woods for
building construction, are less desirable for durable painting than the
lightweight but less abundant softwoods. Paint performance on the heavier
softwoods can be improved by careful selection, application, and maintenance
of the most suitable paints, but it can also be improved with a wider
tolerance for different kinds of paints by any one of several methods of
specially processing lumber or plywood.
The beneficial processing methods discovered thus far depend on more than
one principle, stabilizing the wood against swelling and shrinking, covering the wood with thin layers of uniformly textured material, and mechanically treating the surface to subdivide the troublesome wide bands of
summerwood.
Moisture stability can be imparted by acetylating the wood, or at least the
parts of the wood nearest the painted surface. The method involves problems of chemical engineering that have not yet been completely solved, but
if offers promise of better paint performance without appreciable change
in any of the desirable properties of wood. Stabilization by suitable
impregnation with resins is also effective in improving paint performance,
but it alters the weight, appearance, and toughness of wood and adds
seriously to cost.
Coverings to improve paintability have so far been developed more extensively for plywood than for lumber, though in principle they are applicable to both. Successful coverings usually involve resin impregnation,
which under some circumstances not yet fully understood may tend to
cause abnormal alligatoring of coatings, though it is evident that such
troubles can be controlled when more is known about them. Coverings can
be made of pulpboard, paper, ground wood, and doubtless of other materials
not yet tried. Coverings materially alter the appearance of wood for
use without finishes or with transparent finishes.
Mechanically striating the surfaces of Douglas-fir plywood greatly improves
paint performance and minimizes face checking. The process should be applicable to other woods, including lumber. The striations now used greatly
alter the appearance of wood, though the novel appearance is an advantage
for many purposes. Other methods of mechanical treatment that produce less
alteration of appearance might well be sought.
How far the burden of changing customary practices in the interest of
better paint performance should fall on lumber producers and how far on
-15-
paint makers is a matter for the give and take of industry to determine.
It has now become evident that, from at least the technical point of view,
progress can be made on both sides of the paint-wood interface.
.16-
Appendix
The method used to express the approximate composition of house paints
concisely by suitable symbols was published in 1937 (10). Capital
letters with their subscripts stand for the opaque white pigments and
their proportions in percentages of the total pigment by volume. The
letter L stands for white lead, Z for zinc oxide, T for titanium dioxide,
and S for zinc sulfide. Thus paint L was pure white-lead paint. It
might be written L100 , but the subscript is omitted when it is obvious.
Paint LZ55 would have a pigment composed of 55 percent zinc oxide and 45
percent (100 minus 55) by volume of white lead. In a paint (LZ30)80,
the parentheses indicate that the opaque pigments, white lead and zinc
oxide, together amounted to 80 percent of the total pigment (30 percent
of zinc oxide plus 50 percent white lead). The remaining 20 percent of
pigment in this paint would be extending pigments. When titanium dioxide is present, it is assumed that, unless otherwise indicated, there
are 3.2 volumes of extending pigment for each volume of titanium dioxide.
Thus, paint TL28Z28 was paint with a pigment composed of 28 percent zinc
oxide, 28 percent of white lead, and 44 percent of titanium pigment
(100 less 28 for the zinc oxide, less 28 for the white lead), or 10.5
percent of titanium dioxide (44 divided by the quantity (3.2 plus 1))
and 33.5 percent of extending pigment. But in paint (TL28Z28) 116 the
equivalent opaque pigment was 116 percent, signifying less than the
standard dilution of titanium dioxide with extending pigment; hence, the
content of titanium dioxide was 14.3 percent (116 minus 28 minus 28
divided by the quantity (3.2 plus 1)) and that of extending pigment was
29.7 percent (100 minus 28 minus 28 minus 14.3).
Unless otherwise indicated, the nonvolatile vehicle in house paints is
understood to be a drying oil, such as linseed oil, chiefly in unbodied
(not thickened in viscosity) form. In priming paint TL 29 (e) or in
enamel (TL10Z20 ) 133 (e) much or all of the drying oil was bodied. In an
enamel (TZ25)138(re) the vehicle would contain resin of unspecified kind
and bodied oil, that is, an oleoresinous varnish. In enamel(TZ25)138(pre)
the varnish was one made with phenolic resin. In enamel T 246(ar) the
vehicle was an alkyd resin with no bodied drying oil other than that contained in the resin itself. If bodied drying were added together with
the alkyd resin, the symbol would be (are).
The ratio by volume of total pigment to total nonvolatile material in
a paint, which is also the fraction of the dried coating occupied by
pigment, is indicated by the symbol piny and the appropriate decimal
fraction. Thus p/nv 0.29 means that the paint, as used for the finish
coat, contained 0.29 gallon of total pigment to a gallon of total nonvolatile material.
-17-
Literature Cited
1.
Browne, F. L. and Hrubesky, C. E.
1927 Water-resistant animal glue. Ind. Eng. Chem. 19:215.
2.
Browne, F. L.
1930 Drying of exterior paints under various conditions and over
different woods. Ind. Eng. Chem. 22:400.
1930 Effect of priming-coat reduction and special primers upon
paint service on different woods. Ind. Eng. Chem. 22:847.
4.
1930 Procedure used by the Forest Products Laboratory for evaluating paint service on wood. Am. Soc. for Testing Materials,
Proceedings 30(2):852.
5.
1931 Adhesion in the painting and in the gluing of wood. Ind.
Eng. Chem. 23:290.
6.
1934 Effect of aluminum priming paint on the durability of house
paints on wood. Ind. Eng. Chem. 26:369.
7.
1935 Special priming paints for wood.
Eng. Chem. 27:292.
8.
1935 Effect of change from linoxyn gel to xerogel on the
behavior
of paint. Colloid Symposium Monograph 11:211.
9.
1936 Effect of extractive substances in certain woods on the
durability of paint coatings. Ind. Eng. Chem. 28:416.
10.
1937 A proposed system of classification for house paints. Ind.
Eng. Chem. 29:1018.
11.
1940 Painting and finishing wood, chapter 7 in "Wood Handbook"
of the Forest Products Laboratory, unnumbered publications
of the U. S. Dept. of Agriculture, Supt. of Documents,
Washington, D. C.
12.
1941 The two-coat system of house painting. Ind. Eng. Chem. 33:900.
13.
Browne, F. L.
1937 Wood properties and paint durability. U. S. Dept. of
Agriculture, Miscellaneous Publication No. 629, Supt. of
Documents, Washington, D. C.
14.
1948 Behavior of house paints on different woods, Forest Products Laboratory Report No. 21053.
15. 1949
and Rietz, R. C.
Exudation of pitch and oils in wood. Forest Products Laboratory
Report No. 21735.
16. Erickson, E. C. 0. and Mackin, G. E.
1945 Paper-base laminates offer high strength. Plastics 2(2): 26
and Am. Soc. Mech. Eng. Trans. 67(4):267.
17. St. Paul Test Fence Committee
1934 Priming-coat reductions for painting new wood surfaces. Am..
Paint J. 19(Dec. 10):7
18. Stamm, A. J. and Seborg, B. M.
1936 Minimizing wood shrinkage and swelling. Treating with synthetic
resin-forming materials. lnd. Eng. Chem. 28:1164.
19. Tarkow, H.
1949 Swelling and shrinking of wood, paper, and other cotton textiles
and their control. Tech. Assoc. Pulp and Paper Ind. 32:203.
20.
Tarkow, H. and Stamm, A. J.
1950 Acetylated wood. Forest Products Laboratory Report No. 1593.
Table 1.--The 1947 tests on acetylated wood -- relative performance of
coatings on acetylated as compared with untreated wood after
4-1/2 years' exposure to the weather facing south at
Madison, Wis.
• Relative performance of
Priming paint or
Finish paint
: coating on acetylated Sweetor enamel
enamel undercoating
: gum as compared with un
:
Chief ingre- :Pigment :Chief ingre- :Pigment: treated sweetgum
dients and :volume, :dients and :volume,:
their pro-
: p/nv :their
: p/nv :
portionsl
:proportional •
•
On Ranels faced with sweetgum sapwood
L
: 0.30
:L
: 0.30 :Slightlyg more durable
:Distinctly more durable
TL30(e)
.4o
:(T1,29z30 ) 168
: .29
)
(T1.3_Z__
.19
:(TL__Z__)
: .29 :Distinctly more durable
0 50 168
2Y '0'168
T 133 (e)
:Distinctly more durable
.60
:(TL10Z20)133(e): .40
T 133 (e)
.60
:T 133 (e)
: .40 :No difference up to 4-1/2 years
T 133 (e)
: .31 :Very slightly more durable
: .60
:T139 (pre)
T 133 (e)
.60
:T044(ar)
: .22 :Markedly more durable
:Very markedly more durable
Low-viscosity nitrocellulose lacquer
enamel, white
High-viscosity nitrocellulose lacquer :
:Markedly more durable
enamel4 white
.29 :No difference up to 4-1/2 years
Aluminum
:r
:(-129X3o)168
T 133 (e)
: .60
:(TZ15 ) 35 (ar) : .36 :Very markedly more durable
On panels faced with sweetgum heartwood
a
L
: .30 :L
.30 :Slightly more durable
(TL30Z-o
9
:(TL29Z30)168
:
.29
:No difference up to 4-1/2 years
) )168 : - 1
T 133 (e)
: .60
• .22 :Distinctly more durable
:T 244 (ar)
Low-viscosity nitrocellulose lacquer :
:Very markedly more durable
enamel, white
:
–For the meaning of the symbols for compositions of paints see the appendix
to this paper.
gThe adverbs indicate increasingly significant differences in the following
order: very slightly, slightly, distinctly, markedly, very markedly.
Aluminum house paint made with aluminum powder in bodied linseed oil, resinfree vehicle.
Table 2.--The1932testsoze
ngrnellow
pine bevel siding
Covering :
: Relative performance
Paint coating
on
: (Three coats were applied):
siding • .
Kind of paintl 'Thickness: Durability: Integrity
: after 35
:
: months
:
•
.4
Months
Mile
•
Kraft paper : L p/nv 0.28
None
:
do
: 6.1
:
5.2
35
29
Poor
Bad
Tissue paper:
None
•.
:
:
6.6
22
Badl
5.8
30
Badei-
Kraft paper :(TZ23 ) 92p/nv 0.28: 6.6
None
do
6.o
35
3o
do
do
•
Tissue paper:
None
•
•
do
ad
6.1
25
5.9
25
4
roor•
Bad
Bad2
i Bad
1For the Meanings of the symbold for composition of the paints See
the appendix to this paper.
Thickness of the dried coating was calculated from the spreading
rates by methods previously described (12). One mil is 0.001
inch.
Successive stages in the deterioration in integrity of coatings are
rated good, fair, poor, and bad with the added subdivisions of
high (+), medium, and low (-) in each stage. The time elapsed
until the coating reaches medium poor, when it is considered ready
for repainting, is called the durability (4).
C rumbs or flakes of paint became detached from the surface of the
kraft paper, leaving the paper firmly attached to the wood.
;rumba or scales of paint that became detached took with them part
or all of the tissue paper, that is, the line of separation was
chiefly within the paper.
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Figure 6.--Condition of paint on plain Douglas-fir plywood in the 1944 tests
of plywood covered with resin-impregnated paper (see fig. 7) after exposure
vertically facing south for 71 months at Madison, Wis. The top and middle
rows of four test areas each received a priming coat followed by two finish
coats of paint L; the bottom row of four test areas received a priming
coat followed by one finish coat of paint L. From left to right the priming
coats on the top row of test areas were wood sealer (pre), wood sealer (ar),
aluminum in bodied linseed oil, aluminum in phenolic-resin varnish; on the
middle row paint L with p/nv 0.50, p/nv 0.40, p/nv 0.30, p/nv 0.20; and on
the bottom row house-paint primer TL29 (e) p/nv 0.50, paint L p/nv 0.40,
primer TL29(e) p/nv 0.40, and primer TL 29(e) p/nv 0.30.
ZM 88954 F
57.4
Figure 7.--Condition of paint on plywood covered with resin-impregnated
paper in the 1944 tests after exposure vertically facing south for 71
months at Madison, Wis. The primers and paints and their arrangement
on the test areas were the same as they were on the plain plywood shown
in figure 60
ZM 88951 F
Figure 8.--Superior performance of paint on striated Douglasfir plywood as compared with plain plywood. Both views were
taken on parts of the same building in southern Arizona that
were last painted about 2 years previously with the same kind
of paint. Note that the striated plywood shows no face checking as well as superior coating integrity, whereas the plain
plywood checked very badly.
ZM 90110 F
ZM 90111 F
1